1. Field of the Invention
This invention relates to integrated optoelectronics in general and more specifically to monolithic microlens arrays for photodetector imagers.
2. Description of the Related Art
Optoelectronic arrays such as photodetector imagers are commonly combined with complementary arrays of microlenses to enhance efficiency by concentrating incident radiation into an active photodetecting region. Various techniques have been employed to fabricate the microlens arrays. In some methods, a microlens array is fabricated separately from the photoelectronic chip then bonded to the chip. This requires that the microlens array be aligned properly with the photoelectronic chip during bonding, and that the alignment be accurately maintained during the life of the device. Because such alignment is difficult in mass-production, monolithic fabrication of microlens arrays integrated with the optoelectronics is preferable.
One conventional method of monolithic fabrication of microlens arrays is described by Zoran D. Popovic, Robert A. Sprague, and G. A. Neville Connell in "Technique for Monolithic Fabrication of Microlens Arrays, " Applied Optics, Vol. 27, No. 7, pp. 1281-84 (April 1988). Briefly, their method consists of four steps: first, aluminum film is deposited on a quartz substrate and patterned with 15 micron circular aperture holes. Second, 30 micron circular pedestals are formed on top of the holes. Next, 25 micron diameter, 12 micron high cylinders of photoresist are developed on top of the pedestals. Finally, heating to 140 degrees centigrade melts the pedestals which then form roughly hemispherical droplets, under the influence of surface tension. The formation of hemispherical droplets is analogous to the familiar way in which rain droplets "bead up" on the hood of freshly waxed automobiles.
The above described fabrication method has several drawbacks. First, it fails to approach a 100 percent fill factor for the surface area. Second, when a non-circular lens base is used, the method produces imperfect lens shapes which deviate from spherical, resulting in broadened focal spots.
A further problem is the sensitivity of the reflow method to process conditions. Interfacial adhesion and wetting of the photoresist over the planarizing material are process dependent, and it is therefore difficult to achieve reproducible results. Photoresist lenses for visible imagers are typically fabricated on an optically transparent planarizing layer or over color filters. The lens, filter and planarizing materials have similar surface energies, which makes controlling the wetting and spreading of the photoresist during reflow difficult to control. A narrow range of process conditions must be maintained for success.
The reflow fabrication method is undesirably limited in its ability to produce "slow" microlenses (with small aperture to focal length ratio). Such microlenses have only slight curvature over their aperture, and it is difficult to accurately produce such a shape, as the surface tension causes the edges to rise and the center to sag. Any such sag introduces significant aberration.
Reworking of imperfect reflow microlenses is expensive as it requires stripping of the lens, the planarization layer and any underlying color filter materials (which are commonly added).
The failure of reflow lenses to achieve high fill factor can be easily understood by reference to FIG. 1. The figure shows only four pixels, for ease of illustration, although actual image matrices typically would include hundreds, thousands, or even millions of pixels, as is well known. The pixels are typically laid out substantially as shown, in a rectangular or square matrix with rows 10 and columns 12 at right angles. The round regions 14 represent the microlenses, formed by the reflow method, which occupy area within rectangular cells 16 (shown square, within phantom lines 17). A minimum space 18 is required between the circumference of the microlenses 14 and any adjacent microlenses. If this minimum space is not observed, the lenses 14 will flow together during melting to form larger drops, losing their distinct identities.
In the plan illustrated, it is obvious that each microlens is, in area plan, a round object occupying a square cell. Therefore, even neglecting interlens spacing, full fill-factor can never be achieved, as the area of a circle of diameter d is only .pi./4 of the area of the square enclosing the circle. The situation worsens when the requisite inter-lens spacing is considered; and the fill-factor degrades to an abysmal level as the lenslets are scaled down below ten microns, as the interlens spacing is not correspondingly scaleable: a minimum spacing is required between the lenses to prevent contact of photoresist islands during reflow, and this spacing is generally limited by the photolithographic resolution. With reflow lenses, a typical fill factor of less than 65 percent is achievable for 5 micron square pixel sizes with 1 micron separation.
To increase fill factor, it would be desirable to fabricate arrays of microlenses in which each microlens approximates a polygonal segment of a spherical contour, and the polygonal microlenses are placed contiguously in a tiling pattern to cover the receptive area, for example in a rectangular or square matrix. However, the reflow method cannot fabricate microlenses which have square or otherwise polygonal borders. Consider a pillar of photoresist, which is allowed to melt and reflow to accomodate a non-circular aperture (shown as a square, projecting onto abcd) as shown in FIG. 2. The resulting microlens 20 is non-spherical (and in fact, not rotationally symmetrical about its central axis L). The lozenge-like lenslet has been twice cut and a pie-like wedge removed, to clearly show the curvature of the surface in two different planes. The first cutaway 22 is taken parallel to the square side of the lenslet; the second cutaway 24 is in a plane slicing diagonally across the square aperture, corner to corner.
If the lozenge-like microlens of FIG. 2 is formed by droplet reflow, the surface tension of the reflow droplet will form the microlens surface in a minimum-surface form (constrained by the shape of the square aperture border). Unfortunately, the minimum surface formed by wetting a polygonal aperture is emphatically not a segment of a sphere. This is easily seen in FIG. 2: the arc 28 which bounds section 22 descends from the zenith z to the side of the lozenge 20, with elevation h. The arc 30, makes the same descent, but over a longer run, necessarily longer because the diagonal of a square is always longer than it width. This is not characteristic of a spherical surface (or even a surface with rotational symmetry about a central axis at z). Thus, surface tension does not permit formation of droplets of polygonal borders with spherical surfaces. Aspherical aberrations caused by the square (or generally, polygonal) borders degrade the performance of reflow microlenses in polygonal apertures, by broadening the focal region.